Band 34, Heft 4 -

By means of creep property measurement and microstructure observation, an investigation has been made into microstructure and creep behavior of a directional solidification Ni-based superalloy at high temperatures. Results show that after full heat treatment, small cuboidal γ′ precipitates distribute in the dendrite regions, while coarser ones distribute in the inter-dendrite regions. In the primary stage of creep, the γ′ phase in alloy is transformed into the rafted structure along the direction vertical to stress axis, and then the creep of alloy enters the steady state stage. And dislocations slipping in the g matrix and climbing over the rafted γ′ phase are thought to be the deformation mechanism of the alloy during steady creep stage. At the latter stage of creep, the alternate slipping of dislocations may shear and twist the rafted γ′/γ phases, which promotes the initiation and propagation of the micro-cracks along the boundaries near the coarser rafted γ′ phase. And the bigger probability of the creep damage occurs in the grain boundaries along 45° angles relative to the stress axis due to them bearing relatively bigger shearing stress.

An arc-sprayed composite coating consisted of an under coat Fe25Cr5Al and a top coat Al5Si was sprayed on the reactor wall surface, which is made of mild steel and used for batch production of titanium sponges. After a trial for 15 production cycles, the morphology of the sprayed coating changed greatly into somewhat like a thermal barrier coating. A large number of composite oxides containing aluminum and chromium had been generated in situ within the protective coatings, especially close to the coating/substrate interface. The oxides generated at the interface could improve the bonding of the coating to the steel substrate together with the surface alumina on the coating surface may provide an effective and long-term protection to the steel substrate of reactor at high temperatures.

Molybdenum disilicide based composites containing different types of additive materials such as SiC and nano SiC particles have been investigated, based on the detailed examination of their microstructures and fracture surfaces after creep. The materials were prepared via powder metallurgy using high temperature controlled reaction sintering (CRS). The creep experiments were carried out in four-point bending mode in air in the temperature range from 1273 to 1673 K, with constant stress of 100 MPa. The creep behaviour and the mechanisms of damage were investigated on fracture surfaces by SEM and the microstructure and its response to high temperature load were investigated by TEM using the thin foil technique.

In this study, micron size uniform particles of aluminum compounds were synthesized by the homogeneous precipitation method under reflux boiling, using aluminum nitrate, urea and potassium sulfate as starting materials. In this method, varying amounts of the reactants were heated in a Pyrex glass round bottom flask at the boiling temperature for various periods of time, which led to the production of precipitated particles in the reactant mixtures. Scanning electron microscopic (SEM) examination of the precipitated particles revealed that uniformity in particle morphology of the precipitated particles was dependent on the applied experimental conditions. As such, the latter were extensively optimized for obtaining particles of uniform morphology and size. Some of the batches of the precipitated particles were calcined at the elevated temperature (800 and 1000 °C) for transformation into crystalline solids. Similarly, the as prepared and calcined particles were characterized by SEM, XRD, TGA/DTA and FTIR. XRD results revealed that at 1000 °C of calcinations, the as prepared amorphous alumina transformed in to crystalline state, while below this temperature, i.e. at 800 °C calcination, the powder was amorphous in nature. The point of zero charge (PZC) of the desired sample was estimated by the salt addition method.

Due to combined mechanical and environmental degradation, i.e. coupled creep and carburization, Cr25Ni35Nb alloy often fails prior to the expected design life. In the present paper, based on the continuum damage mechanics, the constitutive model of coupled multi-damage factors for computing the damage evolution of Cr25Ni35Nb alloy was proposed. The damage prediction was carried out by using finite element method based on ABAQUS code. And then damage evolution processes at different operating temperatures (950 °C and 1050 °C) were simulated and the effect of service temperature on the damage evolution was discussed. The results showed that the rate of damage increased obviously with operating temperature increasing. The location with maximum damage is along the inner surface of tube, which implies that fracture begins along the inner surface of tube under the action of coupled creep and carburization damage and this is coincident with the actual observation of the failure of furnace tube.

In this work, synthesis and characterization of composite materials based on NanoClay(NC) and boric acid doped PolyAniline (PANI) is studied. PANI was successfully incorporated into NC to form PANI-NC composites. The resulting organic-inorganic hybrid material, PANI-NC was characterized by various physicochemical techniques. Formation of PANI inside the clay tactoid has been confirmed by X-ray diffraction studies (XRD), scanning electron microscope (SEM) and FT-IR. Also, conductivity and physical properties of the PANI-NC composites were investigated.

The brazing of Mo-Cu composite and 304 stainless steel was carried out in vacuum with Ni-Cr-P filler metal at 980 °C for 20 min. Microstructure in Mo-Cu/304 stainless steel joint was investigated by field-emission scanning electron microscope (FE-SEM) with energy dispersive spectrometer (EDS) and shear strength was measured by shearing test. The results indicate that shear strength of the Mo-Cu/304 stainless steel joint is about 155 MPa. There forms eutectic structure of γ-Ni solid solution with Ni 3 P in the braze seam. Ni-Cu(Mo) and Ni-Fe solid solution are at the interface beside Mo-Cu composite and 304 stainless steel, respectively. Shear fracture exhibits mixed ductile-brittle fracture feature with trans-granular fracture, ductile dimples and tearing edges. Fracture originates from the interface between brazing seam and Mo-Cu composite and it propagates to the braze seam due to the formation of brittle Ni 5 P 2 and Cr 3 P precipitation.

The attraction of solar energy is greatly enhanced by the possibility of it being used during times of reduced or non-existent solar flux, such as weather induced intermittences or the darkness of the night. Therefore optimizing thermal storage for use in solar energy plants is crucial for the success of this sustainable energy source. Here we present a study of a structured bed filler dedicated to Thermocline type thermal storage, believed to outweigh the financial and thermal benefits of other systems currently in use such as packed bed Thermocline tanks. Several criterions such as Thermocline thickness and Thermocline centering are defined with the purpose of facilitating the assessment of the efficiency of the tank to complement the standard concepts of power output. A numerical model is developed that reduces to two dimensions the modeling of such a tank. The structure within the tank is designed to be built using simple bricks harboring rectangular channels through which the solar heat transfer and storage fluid will flow. The model is scrutinized and tested for physical robustness, and the results are presented in this paper. The consistency of the model is achieved within particular ranges for each physical variable.

An analytical approach is proposed to optimize the thickness of petroleum coke for achieving maximum microwave power absorption in microwave heating based on analysis of reflection loss (RL). The microwave RL of the petroleum coke layer was studied over the moisture content range of 1%–5% at 20 °C and the petroleum coke (10% moisture content) in the temperature range of 20 to 100 °C at 2.45 GHz. The results show that RL depends sensitively on the thickness of the petroleum coke and the absorption peak shifts towards a larger thickness as the moisture content of the petroleum coke increases. There exists a matching thickness corresponding to the maximum microwave absorption, the maximum absorbing peak decreases when the thickness of petroleum coke exceeds the matching thickness. We also show that the absorption peak is found to move towards a smaller thickness region with increasing petroleum coke temperature.

The paper presents numerical analysis of the cyclic external fluxes on diffusion process. It is shown, that the cycles characteristics strongly affects the concentration profiles and stress distribution in material. Calculations were performed using the bi-velocity method with the stress field factor. Moreover, the more general model describing the cyclic carburization process influenced by self stress was formulated. Numerical solutions were obtained assuming composition dependent diffusion coefficients. The results of simulation were compared with the experimental data. The model proposed in this paper may be applied to other diffusion controlled processes with the external flux of the diffusive component.

LiNi 0.5 Mn 1.5 O 4 cathode material was synthesized through sol-gel self-combustion reaction method. LiNi 0.5 Mn 1.5 O 4 powders were subsequently characterized as cathode materials in a Li-ion coin cell comprising a Li anode with electrolyte A or electrolyte B. 1.0 mol/L Lithium Hexafluorophosphate (LiPF 6 ) dissolved in volume ration of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) to diethyl carbonate (DEC) corresponded to 4:3:3as electrolyte A, 1.0 mol/L LiPF 6 dissolved in volume ration of EC to EMC to DEC corresponded to 4:2:4 as electrolyte B. Electrochemical performance of lithium cells was evaluated. These tests showed that no matter the cells with electrolyte A or electrolyte B has good discharge platform in 4.7V range (3.5V–4.75V) at the rate of 0.1C, the initial discharge capacity of cell with electrolyte B was higher than that with electrolyte A.

Erosion of thermal barrier coatings occurs when atmospheric or carbon particles from the combustion chamber are ingested into aviation turbine engines. To understand the influence of erosion on the service life of thermal barrier coatings, we introduce the erosion and crack propagation models, and then by using finite element simulations, determine the relationship between the penetrating depth, the maximum principle stress and impingement variables such as velocity and angle. It is shown that cracks nucleate and extend during the erosion process and the length of a crack increases with the increase of the particle velocity and impact angle.

The microstructural change of directionally solidified nickel-base superalloy which was aged at 900 °C for 1500 hours and tensile behavior at different temperatures were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). γ′ phase of the alloy coarsens and rafts in the course of aged treatment. The driving force of rafting is the decrease of interface energy and elastic strain energy. The stress of aged alloy increases slightly with the testing temperature. This arises from a few dislocations shearing the γ′ precipitates. There is a peak stress value at 760 °C, which is attributed to the high strength of the γ′ phase, the homogeneous deformation structure, and dislocation-γ′ precipitate and dislocation–dislocation interactions. The stress then decreases rapidly with increased temperature. The low stress of the γ′ phase and γ′ rafting at high temperature contribute to the drop of alloy strength. The change of elongation is inverse to that of the stress.